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knife edge flight

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Old 01-20-2013, 08:14 AM
  #1
orthobird
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Default knife edge flight

does anyone know what effect being tail heavy has on knife edge flight or inverted flight?
tail heavy by 1 to 2 cm's on an airplane that is 45 pounds and 116 inches long.
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Old 01-20-2013, 09:33 AM
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Default RE: knife edge flight

1-2 cm is not much on that size plane.
What are the symptoms?
On an earobat I find more coupling issues if its noseheavy..

You need excessive rudder deflection?
Pulling to the canopy or gear?
Wing wants to roll out?
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Old 01-20-2013, 10:19 AM
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Default RE: knife edge flight

Mine (a biplane) wants to roll out of the KE


Ameyam

Ed: Sorry orthobird , didnt mean to hijack your thread
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Old 01-20-2013, 10:32 AM
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speedracerntrixie
 
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Default RE: knife edge flight

Usually a tail heavy airplane will tuck to the gear badly in knife. Wanting to roll out or in is a dihidral thing.
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Old 01-20-2013, 10:42 AM
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nitro wing
 
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Default RE: knife edge flight

My small eflite sukhoi wants to roll when I crank on the rudder at cruise speed in KE, it has no dihedral, but likely some other issues going on too.

Ameyam
I got your email, will respond soon.
Bipes have a habit of having coupling, especially lesser known and cheaper ones.
Some of the modern better refined ones do fly alot cleaner
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Old 01-20-2013, 03:06 PM
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Default RE: knife edge flight

inverted flight it will fly level or climb depending on the amount of TH ness
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Old 01-20-2013, 10:04 PM
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Default RE: knife edge flight

i've found tail heavy will usually tuck to the belly and roll out of KE flight.
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Old 01-22-2013, 01:18 PM
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Default RE: knife edge flight

I find much depends on the design. Fuse area ahead and behind the CG as well as area distribution above and below wing centerline play roles in KE performance (roll and pitch coupling).

In general, more aft CG gives more tail control response (both rudder AND elevator). Also, it's common to change elevator trim as you change CG. The elevator / CG trim relationship is important to KE for this reason:

You'll find as you move the CG back, you need less "UP" elevator to maintain level flight (and conversely less down control needed to maintain inverted level flight). With this in mind, if you roll the plane to KE attitude, as the CG moves aft, your elevator downward trim tendency will mean the plane will probably "tuck" more to the gear side (wich is good if the plane formerly pulled to the canopy).

Make sense?
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Old 01-22-2013, 01:36 PM
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Default RE: knife edge flight

very good wind J (explaining that deal to all)
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Old 01-24-2013, 05:52 PM
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Default RE: knife edge flight

ok, this is all starting to make sense
on the last flight, i was able to make it knife edge pretty nice
this is what i did:"
rolled it on its side, then applied rudder to opposite direction of the roll
then i noticed plane wanted to tuck to gear, and i pulled elevator stick down slightly, meaning, giving it "up" elevator.
this was done, after i flew plane level, and gave plane two clicks down elevator trim, then plane flew more level (somehow),
then when i did inverted flight, it required less "down" elevator input than before, to fly level inverted.
then i flew "low" inverted, which was cool,
then did knife edge (not low) with applying the bit of "up" elevator to make it go straight.

does this seem or mean that my plane is "tail heavy"?
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Old 01-24-2013, 06:45 PM
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Default RE: knife edge flight

We should just start from simple basics..

Before flying, set up your plane on your bench or ground.
Raise the tail until you get a 0 degree reading on a robart incidence meter or similar when measuring the wings LE to TE center. Then take a horizontal stab measurement.

I think you have an Extra, which should have a 0 reading from wings to stabs. Both must read 0 each side. wing/stab root and outer tips, check 10x
Turn radio on and and carefully examine your control surfaces from last flight. I asume you had it trimmed to fly as neutral as best possible.
Write down any trim that is not centered and record radio trims also.

The engine should also be at 0 degrees to wing and stabs, this is harder to measure and you maybe able to get away with leaving it for now.
If your wings and stabs are not 0 to each other, then this must be modified, you cannot counter trim this and expect good flights.

If they are all at 0, then check the controls and try to center everything manually via linkage adjustments, after the servos have been centered per radio.
All subtrim and EPA should be at 0, not -30 or plus 20....
Get the linkages and servo horns as close as possible to neutral, then adjust subtrim and EPA. Values below 10 are ideal.

Add 20-30% expo all around except throttle for a start. Depending on radio brand its + or- values. The more throws you have the more expo is required. Full on 45 or more 3D throws usually have 60% expo, or it will be extremely sensitive to fly.


Try this and fly it, then report back

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Old 01-24-2013, 06:58 PM
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Default RE: knife edge flight

Your flight sounds OK, and trims sound as expected.

Try a nice 60-80% level power pass over runway and climb into a 45 with good speed, roll inverted and neutralize sticks.
The plane should keep its heading and slowly arc back down,
If it actually climbs hands off ( unusual), it would be quite tailheavy.

Cruise level and roll inverted, if you just need a touch of down elevator, you are likely in a good range. If it requires a steady down elevator input because it wants to hit the runway, you are nosesheavy.

This only applies after you have manually trimmed and adjusted incidence and servo travel on the ground.

Your CG is likely close as you are flying it, after all.
Do the other stuff before making any CG changes.
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Old 01-25-2013, 04:07 PM
  #13
orthobird
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Default RE: knife edge flight

thank you so much nitro, all great advice.  will do it!!
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Old 01-25-2013, 06:01 PM
  #14
nitro wing
 
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Default RE: knife edge flight

not sure if the robart incidence meter can read surfaces of your size, check around to see if a larger one is available. The incidences are your starting point, everything can be set from there.
Good luck
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Old 01-26-2013, 04:55 AM
  #15
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Default RE: knife edge flight

ortho, consider that your elevator becomes your rudder (effectively) when the plane is rolled 90 degrees? Any trim you have on the elevator at that point will turn the plane? Thus, you want to trim the plane's CG for a neutral elevator....

The alternative to trimming the CG is to use your radio's rudder to elevator mixing capabilities?
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Old 01-26-2013, 09:52 AM
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Default RE: knife edge flight

He will likely end up mixing in some EL on KE, he just needs to get the aircraft checked and trimmed first.

Ortho, I will post the engine thrust check, its another important step for clean flights, it can cause all kinds of unwanted trim behaviour if not set up well, and goes hand in hand with incidence checks.

This should keep you busy for a bit, its worth following all this for sure.

http://www.dream-rc-airplanes.com/trimming.html

http://www.rcflightschool.com/Inter_...ter_0A4-13.pdf
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Old 01-26-2013, 12:30 PM
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Default RE: knife edge flight

Poorpusing is a factor being tail heavy as i found out !!!
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Old 01-26-2013, 07:54 PM
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orthobird
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Default RE: knife edge flight

if you do not mind me asking, whilst i do not mean to laugh when i read this, but can you tell me what is poorpusing?
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Old 02-04-2013, 04:04 AM
  #19
orthobird
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Default RE: knife edge flight

i flew 3 of my planes yesterday
and 2 of them would tuck to gear when KE, i would have to add Up elevator to get it to fly straight.
so i will program a mix elevator to rudder.
thank you all
i will update you all on the next flight.
BTW, a GP yak that i flew yesterday 3 times, when i got home, i noticed the rudder hinges came loose!!!
Ouch, must have been all that stress from KE attempts.
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Old 02-04-2013, 05:39 AM
  #20
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Default RE: knife edge flight

Usually doesn't take much. Maybe 2-3%?
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Old 02-04-2013, 08:37 AM
  #21
nitro wing
 
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Default RE: knife edge flight

that be a good start, or else the thing will be screaming over the pitt area, wandering all over...
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Old 02-28-2013, 04:27 PM
  #22
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Default RE: knife edge flight

Porpoising is what they meant. I.E. - will not fly straight and level, will climb a bit then dive a bit then climb a bit then dive a bit, like a porpoise.
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Old 03-02-2013, 07:43 PM
  #23
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Default RE: knife edge flight

Violently moves up and down with litlle stick movement no matter how much expo you have !
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Old 03-02-2013, 08:47 PM
  #24
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